The present invention relates to a method of producing a plastic sheet with porous surface. The plastic sheet produced by the method of the present invention is suitable for use as a synthetic paper for printing characters and images thereon by a press or a printer. The plastic sheet, with its substrate removed, is suitable for use as a porous synthetic film for filtrating and separating various substances.
Nowadays, synthetic papers made of plastics have spreading use by virtue of their waterproof nature and other superior properties. Such synthetic papers are produced by a method such as that disclosed in Japanese Patent Publication No. 25430/1974.
In recent years, new types of printers such as ink jet printers and thermal-transfer printers have been developed and put into practical use. It is known that sheets with smooth surface, such as synthetic sheets, are preferably used for obtaining high degree of clarity of the characters and images printed by such new types of printers.
The synthetic sheet produced by the method disclosed in Japanese Patent Publication No. 25430/1974 however, involves a problem in that, when it is used for printing by a thermal-transfer type, characters printed thereon tend to be distorted as if the ink flowed on the sheet.
This is attributable to the fact that, due to small porosity of the surface layer, the synthetic sheet exhibits only a small absorbability.
Through an intense study, the present inventors have succeeded in developing a novel method of producing a plastic sheet with a porous surface layer having a high porosity.